Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixel cells or pixels as an image sensor, each cell including a photosensor, which may be a photogate, photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imaging devices of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc.
One problem experienced in film and solid state cameras is vignetting (i.e., lens shading). Vignetting is a phenomenon of a gradual reduction of image brightness in the periphery of an image as compared to the image center. The light fall-off or darkening towards the edges of the image is inherent in optical lenses and is more pronounced with wide angle lenses. In solid state imaging devices (e.g., digital cameras), the photosensor may also introduce additional unwanted vignetting which is affected by many factors such as, for example, microlens placement, photosensor layout, and depth of the photon well. This additional vignetting effect is more complex than optical vignetting and may have directional non-uniformity and minor local variations. Additionally, there are other causes of vignetting such as, for example, physical blockage of the light path within the camera.
As illustrated in FIG. 1, light passes through a lens 10 and illuminates a pixel array 20. Optical vignetting may be approximated by the “cosine fourth” law, where light fall-off is roughly proportional to the fourth power of the cosine of θ (i.e., cos4(θ)), where θ is the off-axis angle with respect to the pixel array 20. In compact cameras and cameras with wide angle lenses, optical vignetting is more pronounced with an increasing off-axis angle. One known method of vignetting correction utilizes a look up table of correction factors for each pixel in an image. A calibrating image is obtained by imaging a known flat-field. An ideal image is then obtained by selecting ideal pixel values for the known flat-field. The value of each entry of the look up table is derived by dividing the ideal flat field pixel value from the ideal image by the corresponding calibrating pixel value from the calibrating image. Vignetting correction is implemented by multiplying each pixel value of an image by the corresponding factor in the look up table. However, utilizing a look up table requries storage locations capable of storing high precision fractional correction values for each pixel in the image. The storage requirement of this known method of vignetting correction may not be desirable or cost effective. Accordingly, a low cost method of vignetting correction that minimizes storage requirements is needed.